With the advancement of science and technology, improvements in product productivity and the quality control of materials have recently been considered important. In view of such considerations, the measurement of material properties is very important, and various measurement methods for properties such as the impedance, resistance, and density of materials have been examined. For example, the four probe method (JIS K 7194-1994; Testing Method for Resistivity of Conductive Plastics with a Four-Point Probe Array) has been known as a method for measuring the resistance of a material. In the four probe method, four needlelike probes (electrodes) arranged in substantially a line are brought into contact with a test piece, and the resistivity of the test piece is obtained from a direct current applied between the outer two probes and the potential difference between the inner two probes. Note that the measurement method described in JIS K 7194 is a method for measuring the resistivity of a plate-like test piece, and does not relate to a method for measuring the resistance of a powder.
A direct current is generally used for measuring the resistance of a powder. “Correct Measurement and Evaluation of Electrical Characteristics” (Technical Information Institute Co., Ltd, and NPC, pp. 130 to 131 (2005)) describes a method for measuring the powder resistivity of carbon nanotubes (CNT). In this measurement method, a predetermined amount of CNT is packed into a measurement cell and a pressure is applied thereon in stages to evaluate the behavior of the powder under certain CNT densities. Then, the direct-current resistance of CNT is measured while changing the density of the powder.
In measurements using a direct current, however, the resistance component between particles and the resistance component inside particles cannot be separated. Thus, there is limitation in the amount of information obtained regarding powder properties. Also, there are disadvantages in that because a direct current resistance is measured based on Ohm's law, the resistance changes due to Joule heating, and the measurement value is not stable.
On the other hand, measuring the resistance of powder by an alternating current impedance method has also been proposed. In this case, generally, powder is sintered or pressed in advance to form a molded product therefrom, and the resistance is measured. By using the molded product for the measurement, variation in measurement values due to changes in powder density can be prevented.
In the above case, however, because the molded product is used for the measurement, no discussion can be made regarding the relationship between density and resistance. Furthermore, because of the poor reproducibility of the measurement values, it is difficult to obtain highly reliable measurement results.